Illumination varies greatly both across parts of a natural scene and as a function of time, whereas the spectral reflectance function of surfaces remains more stable and is of much greater relevance when searching for specific targets. This study investigates the functional properties of postreceptoral opponent-channel responses, in particular regarding their stability against spatial and temporal variation in illumination. We studied images of natural scenes obtained in UK and Uganda with digital cameras calibrated to produce estimated L-, M-, and S-cone responses of trichromatic primates (human) and birds (starling). For both primates and birds we calculated luminance and red-green opponent (RG) responses. We also calculated a primate blue-yellow-opponent (BY) response. The BY response varies with changes in illumination, both across time and across the image, rendering this factor less invariant. The RG response is much more stable than the BY response across such changes in illumination for primates, less so for birds. These differences between species are due to the greater separation of bird L and M cones in wavelength and the narrower bandwidth of the cone action spectra. This greater separation also produces a larger chromatic signal for a given change in spectral reflectance. Thus bird vision seems to suffer a greater degree of spatiotemporal "clutter" than primate vision, but also enhances differences between targets and background. Therefore, there may be a trade-off between the degree of chromatic clutter in a visual system versus the degree of chromatic difference between a target and its background. Primate and bird visual systems have found different solutions to this trade-off.
It has been suggested (Tadmor and Tolhurst, 1994 Vision Research 34 541-554) that the psychophysical task of discriminating changes in the slope of the amplitude spectrum of a complex image may be similar to detecting differences in the degree of blur. It has also been suggested that human observers may perform this discrimination by detecting changes in the effective contrast within single narrow spatial-frequency bands, rather than by detecting changes in the slope per se which would involve the use of contrast information across many different frequency bands. To distinguish between these two possibilities, we have developed an experiment where observers were asked to discriminate changes in the spectral slope while different amounts of random contrast variation were introduced, with the purpose of disrupting their performance. This disruptive effect was designed to be particularly manifest if the observer really was performing a single-frequency-band contrast discrimination but to be unnoticeable if the observer was discriminating the change of slope per se. Our results imply that the observers do not usually detect changes in contrast in just one narrow spatial-frequency band when they discriminate changes in the slope of the amplitude spectrum. Rather, they must compare contrast between bands or, at least, they use contrast information from more than one band. However, for edge-enhanced (whitened) pictures, there is some evidence to suggest that observers rely on contrast changes in only a limited low-spatial-frequency band.
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